The usage share there referred to stats from Mozilla and Firefox 36, where HTTP/1.0 is seen in 1% of all HTTP responses compared to 10% for HTTP/2. / Daniel - author of those slides
If the GP post is correct, ironically the HTTP/1.0 traffic thats really happening might be services/appliances and (lib)curl (@bagder is principle dev of curl, for those that don't know), not Firefox or any consumer browser.